Added pullup/down controls and a cycle-based wait function to match their specification (150-cycle wait)
Added warning about function call overhead from C
Added atag structures and linkedlist-style "next tag" inline function
Added support for adding/removing events to be detected on a pin
Merge branch 'working_gpio' - fully implemented access to the GPIO registers

This does not use any interrupts; it is only simple code exposing the memory mappings